Circles: Special Segments in Circles

Radius and Diameter

If we know the radius of a circle, it is easy to find the diameter of the circle. 

circle with diameter labeled d made up of two radii, both labeled r
Circle

The diameter of a circle is twice as long as the radius.

d = 2r


If we know the diameter of the circle, we can work backwards to find the radius.  The radius of a circle is one-half as long as the diameter or the diameter divided by two.

The radius of a circle is one-half as long as the diameter or the diameter divided by two.

r equals one-half; d = one-half


Example 1:

If the radius of a circle is 6 inches, how long is the diameter?

Solution:

d = 2r = 2(6) = 12 inches

Example 2:

Find the radius of the following circle.

circle with diameter 9 centimeters
Circle

 

Solution:

d = 9cm

r equals one-half times 9 equals 4 point 5